Different chucking devices are widely used to hold semiconductor wafers or other substrates during processing. Mechanical chucks, for example, can secure a work-piece by using arms or clamps to press the work-piece against a supporting surface. However, the clamping force from a mechanical chuck is inherently non-uniform, resulting in uneven stresses in the work-piece that can cause deformation and uneven thermal contact between the work-piece and the support.
Vacuum chucks secure a work-piece by evacuating a void beneath the work-piece backside, thereby generating a clamping force due to the pressure differential between the processing chamber and the work-piece backside. Vacuum chucks can provide a more uniform clamping force than can mechanical chucks, but in the low pressure environments required for many semiconductor processing applications, the pressure differential is insufficient to generate an adequate clamping force.
Electrostatic chucks provide improved clamping uniformity in vacuum systems. An electrostatic chuck (ESC) uses an electrostatic potential to hold a work-piece in place during processing. By clamping the work-piece to the chuck, improved thermal conductivity between the work-piece and the chuck can be provided. Optionally, a high thermal conductivity gas such as helium may be disposed between the work-piece and the chuck in order to improve heat transfer between the work-piece and the chuck. Examples of mechanical clamps and ESC substrate holders are provided in commonly-owned U.S. Pat. Nos. 5,262,029; 5,880,922 and 5,671,116. As disclosed in U.S. Pat. No. 4,579,618, substrate holders in the form of an electrode can supply radiofrequency (RF) power into the chamber.
A difficulty that arises with the use of an ESC is removal of the residual electrostatic force between the work-piece and the chuck in order to remove the work-piece from the chuck. This residual force results from the accumulation of electric charge at the interface between the work-piece and the ESC support surface. Several techniques have been developed for removing or de-chucking a work-piece. For example, both the electrode and the work-piece can be grounded or, alternatively, the polarity of the chucking voltage applied to the electrode can be reversed in order to discharge the electrode. However, these techniques are not completely effective at removing all the charge on the electrode and the work-piece. A mechanical force is often required to overcome the residual attractive electrostatic force, which can damage the work-piece or create difficulty in retrieving the work-piece from an unintended position. Approaches for de-chucking work-pieces are disclosed in U.S. Pat. Nos. 5,117,121; 6,125,025; 6,236,555 and 6,430,022.
Processing apparatus that use resilient clamping members to engage a substrate with a substrate support surface are disclosed in U.S. Pat. Nos. 4,685,999; 5,266,527 and 5,925,226. Cryogenic cooling systems for processing semiconductor substrates are disclosed in U.S. Pat. Nos. 6,431,115 and 6,695,946.
Despite the developments to date, there is an interest in efficient, low cost apparatus for supporting wafers during processing. It would be an improvement to provide a substrate support that supported a substrate such as a semiconductor wafer while providing good process uniformity without introducing the difficulties associated with de-chucking.